1. Field of the Invention
The present invention relates generally to micro-lenses for imager devices and to methods for producing the same.
2. Brief Description of the Related Art
Image sensing devices (i.e., image sensors) are known that employ light detecting elements (e.g., photo sensors) for use in various applications such as in semiconductor devices. Such image sensors may be formed using a variety of fabrication techniques. Currently, two commonly fabricated image sensors are CMOS image sensors and charge coupled device (CCD) image sensors. Each generally includes an array of pixels containing the photo sensors. The image sensors typically use photo sensors in the form of photogates, phototransistors or photodiodes.
When an image is focused on the imager array, light corresponding to the image is directed to the pixels, including the photo sensors. It is known in the art to use an imager array of pixels having a micro-lens array that includes a convex micro-lens for each pixel. Each micro-lens may be used to direct incoming light through a circuitry region of the corresponding pixel to the photo sensor region, thereby increasing the amount of light reaching the photo sensor and thereby increasing the fill factor of the pixels. Each photo sensor is located at a suitable depth in the image sensor such that light, when collimated and focused by a corresponding micro-lens of a given focal length, is directed onto the photo sensor. Examples of other uses of micro-lens arrays include intensifying illuminating light from pixels of a non-luminescent display device (such as a liquid crystal display device) to increase the brightness of the display, forming an image to be printed in a liquid crystal or light emitting diode printer, and providing focusing for coupling a luminescent device or a receptive device to an optical fiber.
Typically, each pixel in the imager array produces a signal corresponding to the intensity of light impinging on the photo sensor associated with that pixel. The magnitude of the signal produced is approximately proportional to the amount of light impinging on the photo sensor. The signals may be used, for example, to display a corresponding image on a monitor or to otherwise provide information about the optical image.
As semiconductor devices and the corresponding image sensors are made smaller, the micro-lens arrays must be made smaller as well. As each micro-lens is made smaller, the focal length of the micro-lens is reduced. This reduced focal length causes light to be focused at a reduced depth in the image sensor device. Typically, in prior art devices, the depth of the photo sensors is reduced in the substrate to correspond to the reduced focal length of the micro-lenses. Decreasing the photo sensor's depth in the substrate, however, requires manufacturing changes, and results in additional effort, expense and time in processing.
Known fabrication methods for micro-lenses use patterning and reflow techniques that are typically used to produce convex micro-lenses. These fabrication techniques are difficult to tailor to produce micro-lenses that direct light to a particular focal point when the micro-lens size and geometry (and resultant focal length) change.
Additionally, in known image sensors, a large amount of light incident on the image sensor device is not directed to the photo sensor due to the geometry of the micro-lens array. In particular, light incident on the space between individual micro-lenses (the lens-lens space) remains uncaptured by the micro-lens, and never impacts the photo sensor. It is difficult to produce image sensor micro-lenses with improved light capture using known patterning and micro-lens reflow processes.
Further, in known image sensors, a passivation layer such as a silicon nitride passivation layer separates the color filters from the photo sensors. This passivation layer causes undesirable reflection in the optical path to the photo sensors.
Accordingly, there is a desire and need for an image sensor having micro-lenses formed to focus light onto the photo sensors of the image sensor such that the depth of the photo sensors does not have to be decreased even when the image sensor device, including its photo sensors and micro-lenses, decreases in size.
There is also a need for an improved method of fabricating an image sensor micro-lens of desired geometry and focal length. Furthermore, there is a need for an image sensor including micro-lenses having improved light capture. Moreover, there is a need for an image sensor and method having reduced reflection in the current stack of the image sensor.